Multi-step air ring for tubular film process

ABSTRACT

In the extrusion of a thermoplastic resin through a ring die to form a tubular film wherein the film is partially cooled by means of a cooling gas b the tube is drawn over a cooling mandrel, the improvement of using multistepped adjustable rings to direct the cooling gas onto the external surface of the film between the die-gap and the mandrel, thus defining a series of orifices between the rings and the film to effect a gradient in pressure from the die to the mandrel.

June 25, 1974 H c. NORTH HAL 13,819,790

MULTI-STEP AIR RING FOR TUBULAR FILM PRO CESS 5 Sheets-Sheet 1 OriginalFiled Dec. 22

INVENTORS HOWARD C. NORTH GENE C. CALDERWOOD BY CHARLES W. WlLLlAMSON vM. AGENT June 25, 1974 3, NORTH ETAL 3,819,790

MULTI-STEP AIRRING FOR TUBULAR FILM PROCESS Original Filed Dec. 22, 9695 Sheets-Sheet 2 2.. .i X? 1 I INVENTOR HOWARD c. NORTH GENE c.CALDERWOOD CHARLES W. WILLIAMSON I BY MGM? AGENT June 25, 1974 Q'NORTHETAL MULTI'STEP AIR RING FOR TUBULAR FILM PROCESS Original Filed Dec.22, 1969 5 Sheets-Sheet 5 N %N 7 V u OHWM w TTR T i574 \LL m a? =m BM M.3 DW c J F \NQ OHM W I HGC M Q 3 W L" m E m .q Q a g M g Q June 25,1974 H. C. NORTH L MULTI-STEP AIR RING FOR TUBULAR FILM PROCESS OriginalFiled Dec. 22, 1969 5 Sheets-Sheet 4 INVENTORS HOWARD 0. NORTH GENE C.CALDERWOOD CHARLES W. WILLIAMSON AGENT June 25, 197% H. c. NORTH T ALMULTI-STEP AIR RING FOR TUBULAR mm PROCESS 5 Sheets-Sheet 5 OriginalFiled Dec. 22, 1969 INVENTORS HOWARD c. NORTH GENE C. CALDERWOOD CHARLESW. WILLIAMSON AGENT United "States" Patent Office 3,819,790 PatentedJune 25, 1974 3,819,790 MULTI-STEP AIR RING FOR TUBULAR FILM PROCESSHoward C. North, Westfield, and Gene C. Calderwood,

Basking Ridge, N.J., and Charles W. Williamson, Seabrook, Tex.,assignors to Esso Research and Engineering Company Continuation ofabandoned application Ser. No. 887,270, Dec. 22, 1969. This applicationMay 8, 1972, Ser. No.

Int. Cl. B29d 23/04 U.S. Cl. 264-89 7 Claims ABSTRACT OF THE DISCLOSUREIn the extrusion of a thermoplastic resin through a ring die to form atubular film wherein the film is partially cooled by means of a coolinggas before the tube is drawn over a cooling mandrel, the improvement ofusing multistepped adjustable rings to direct the cooling gas onto theexternal surface of the film between the die-gap and the mandrel, thusdefining a series of orifices between the rings and the film to efiect agradient in pressure from the die to the mandrel.

This is a continuation of application Ser. No. 887,270, filed Dec. 22,1969, now abandoned.

CROSS REFERENCES TO RELATED PATENTS AND APPLICATIONS This inventionrelates to the process and apparatus for preparing film fromthermoplastic resins described in U.S. Pats. 3,400,184 and 3,450,806,the disclosures of which are hereby incorporated in this application byreference. A further improvement in the method and apparatus forwater-cooling the film is disclosed in copending application U.S. Ser.No. 853,381, filed Aug. 27, 1969, now U.S. Pat. No. 3,685,576, issuedAug. 22, 1972.

FIELD OF THE INVENTION This invention relates to an apparatus andprocess for preparing film from thermoplastic resins. More particularly,this invention is concerned with the structure and use of a plurality ofrings for directing a stream of cooling gas against the surface of atubular film of a thermoplastic resin, issuing in the molten state froman annular die, before the film engages and is drawn over a sizing andcooling mandrel. Use of the multi-stepped rings disposed in an echelonarray, with the inner edges of the rings being substantially in linealalignment, creates a series of pressure drops of diminishing scope ateach successive lower ring and permits the production of film havinghigher diegap diameter to mandrel-diameter ratios and differentials,faster draw-down to permit higher die-gap to finished film thicknessratios and superior clarity.

PRIOR ART The use of a simple air-ring for externally cooling a tubularfilm of molten thermoplastic resin issuing from an annular orifice ordie while at the same time maintaining the film in an inflated state byuse of an internal pressure of air, before folding, slitting or drawingthe film over a cooling mandrel is old in the art. For example, BritishPat. 943,283 (4-XII-63) shows an internally bafiled airring having asingle annular outlet for directing a stream of cooled air against theouter surface of an inflated tubular film issuing in an upward mannerfrom a rotating die. Closely allied to the present invention is thesimple airring designated as 29 in FIGS. 11 and 14 in U.S. Pat.3,400,184.

SUMMARY We have now found that tubular film of improved clarity may beproduced at higher speeds if the multistepped air-ring of the presentinvention is used with the type of equipment shown in U.S. Pats.3,400,184 and 3,450,806 instead of the simple air-ring described inthese patents. The present invention consists of a plurality of nestedrings sealed against inter-ring leakage, which are adjustable verticallyby means of cams and cam followers to form a conical array or echelon.Attached to each ring is an orifice plate which defines a circularopening for each ring. The entire assembly which is disposed between thedie and mandrel is constructed so that the smallest orifice is equal toor larger in diameter than the die and the largest orifice is equal toor smaller in diameter than the diameter of the mandrel.

In use, the rings are adjusted vertically so that the inner edges of theorifices fall substantially on a straight line projected through themand lie outside a straight line projected from the die-gap to the topedge of the mandrel. While the inner edges of the orifices will fall ona straight line under normal conditions, variations in feed,temperature, draw-down speed and the like may make it desirable todepart from a straight-line array and have the edges of the orificeplates fall on an are projected through them. Such an arc may be convexor concave to the center line of the die but in no case will the sagittaexceed 7.5% of the length of the chord subtending the arc. Under theseconditions, the cooling air which issues from the ring above the toporifice plate and is directed downwardly against the film issuing fromthe die to successively lower orifice plates, creates a series ofpressure drops across each BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows ageneral view of the disposition of a die, the multi-step air-ringscomprising this invention, cooling mandrel, water bath and extrudatefrom the die being directed by the air-rings and drawn over the coolingmandrel and through the water bath in a downward direction.

FIG. 2 shows an enlarged section through a portion of FIG. 1 in greaterdetail.

FIG. 3 is a detailed section through the die and airrings correspondingto a section on line 3-3 of FIG. 4.

FIG. 4 is a partial plan view of the air-rings showing the ringcontrols.

FIG. 5 is a sectional view along line 5-5 of FIG. 4 showing the rings inraised position and the control handle for the middle ring.

FIG. 6 is a sectional view on line 6-6 of FIG. 4 showing the controlhandle for the inner ring.

FIG. 7 is an enlarged section through a portion of the outer ring, camring and middle ring showing the 0 rings and spring biased spacingbuttons.

FIGS. 8 and 9 are sections along lines having the same indicia on FIG.4.

FIG. 10 is an exploded view of a segment of the outer ring, cam ring,middle ring and inner ring showing the means of attachment ofring-control handles, clearance slots for the handles, the cam tracksand cam followers.

FIG. 11 shows a detail of a typical angle track and handle clamp.

FIG. 12 is a plan view of the cam ring showing the angular displacementof the cam slots and handle clearance slots.

DETAILED DESCRIPTION OF DRAWINGS FIG. 1 is a view in elevation of thedie, consisting of an internal die cap 10 and external die adjustingring 12, forming a die-gap 8 from which molten thermoplastic film isextruded to form an upper film bubble 20. The film is drawn down overthe mandrel 50 through waterbath assembly 16 to form a lower film bubble18. Air injected through inlet pipe 30 maintains the upper film bubbleat the proper degree of inflation and air injected through inlet pipe 32does the same for the lower film bubble.

The multi-stepped air-ring assembly 14, situated between the die andmandrel, directs a stream of air against the entire length of theexternal surface of the upper film bubble and serves to counterbalancethe internal pressure, partially cool and solidify the film, but notbelow the temperature at which crystallization occurs, and direct thefilm onto the mandrel.

FIG. 2 is an enlarged section through a portion of FIG. 1 showing thedie, air-ring, mandrel and Water bath in greater detail. The multi-ringassembly 14 comprises an outer plenum chamber wall and mounting ring 44to which is attached a mounting plate 42 for the multistep rings. Aninner plenum chamber wall 72 and inner plenum chamber plate 74 define anair plenum chamber 46 through which air is fed to the rings by annulus76. Cam ring 24 attached to mounting plate 42 carries outer air-ring 22and orifice plate 78, middle air-ring 26 and orifice plate 80 and innerair-ring 28 and orifice plate 82. The height of each ring and itsattached orifice plate is independently adjustable by handles shown indetail in figures which follow. While not the subject of the instantinvention, the construction of the mandrel 50 and the water bath 16 areshown in detail so as to illustrate the best mode of operation of theinvention.

FIG. 3 is a section along line 33 of FIG. 4 showing construction of therings, particularly the cams in camring 24 and the cam followersattached to each adjustable ring. The cam ring and the adjustable ringshave sufficient clearance relative to the contiguous faces of adjacentrings so as to permit each ring to be readily turned without binding.Face-to-face contact which might cause binding is avoided by the use ofa plurality of spring-biased spacing buttons 100 disposed about theinner face of the outer ring and the outer faces of the inner and middlerings. Sealing between the various rings is accomplished by the use ofelastomeric rings 110. Inner ring 28 is adjustable in a verticaldirection by means of handle 84 (shown in FIG. 6), lateral movement ofwhich causes the ring to rotate and cause a plurality of cam followers98 secured around the periphery of ring 28 to move in a plurality of camtracks 92 milled in cam ring 24. A plurality of cam tracks 90 milled incam ring 24, which are angularly displaced from cam tracks 92accommodate the cam followers for both the middle and outer rings. Camfollowers 96 secured around the outer periphery of the middle ringengage cam tracks 90 and cause the middle ring 26 to be raised orlowered when the ring is rotated by means of handle 86 (shown in FIG.Similarly, cam followers 94 secured around the inner periphery of outerring 24 engage cam tracks 90 and cause the outer ring to be raised orlowered when the outer ring 22 is rotated by means of handle 88 securedto it. Handle clamp 118, which is slidable on handle 86, engagesangle-track 116, which is secured to mounting plate 42 and clamps thehandle after positioning.

FIG. 5 referred to above shows how the middle ring handle 86 passesthrough clearance slot 106 in the outer ring and clearance slot 104 inthe cam ring and is attached to middle ring 26. Handle clamp 118 engagesangle-track 114 which is secured to mounting plate 42 and fixes thehandle after positioning.

FIG. 6 referred to above shows how the inner ring handle 84 passesthrough clearance slot 106 in the outer ring and clearance slot 102 inthe cam ring and is attached to inner ring 28. Handle clamp 118 engagesangle-track 112, which is secured to mounting plate 42 and fixes thehandle after positioning.

FIG. 4 is a fragmentary plan view on line 44 of FIG. 3 showing thecontrol handles for the rings, their angular displacement and the limitsof movement.

FIG. 7 is an enlarged section through a portion of the outer ring, camring and middle ring detailing the 0 rings 110 and spring-biased spacingbuttons. The buttons may be made of any material which will glidereadily over the steel or aluminum alloy from which the cam andair-rings are nominally fabricated. Suitable materials are bronze,babbit metal, nylon, polyphenyl carbonates available commercially asLexan, polyfluoroethylenes available commercially as Teflon andpolyphenylethers available commercially as PPO.

FIG. 8 is a section through FIG. 4 at points 88 showing the cam track90, for the cam followers 94 and 96 attached to the outer ring 22 andthe middle ring 26, respectively.

FIG. 9 is a section through FIG. 4 at points 9-9 showing inner ringhandle 84 attached to inner-ring 28, clearance slot 102 in cam ring 24for handle 84 and cam track 92 for cam follower 98.

FIG. 10 referred to above shows in perspective the arrangement of theoperative elements for the rings.

FIGS. 11 and 12 show details as indicated above.

While not wishing to be bound by any theory of operation, it is believedthat the rings co-operate to produce a gradient in pressure drop ofdiminishing scope as the film descends from the die to the mandrel andact as steering guides to maintain uniformity in shape, size andthickness of the upper film bubble.

It will therefore be apparent to those having skill in the art thatvariations in the ring structure and alternative means for attaining agradient in pressure along the external surface may be used withoutdeparting from the scope of the invention as disclosed. For example, thecams and cam followers may be replaced by multi-thread screws having asharp helix angle as a means for raising and lowering the orifice platesattached to the rings. In like manner, once a prototype has establishedthe correct size and position of the orifices for a given size of dieand mandrel, a ring assembly having a plurality of fixed plates or astepped cone instead of adjustable rings may be used for a given resin.Similarly, the single air inlet and the rings may be replaced with aseries of air inlets having approximately the same diameter anddisposition of the orifice plates with individual pressure regulation oneach inlet so as to provide a gradient in pressure from the highestinlet to the lowest. The contribution to the advancement in the art ofproducing film from thermoplastic resins is best illustrated by thefollowing examples.

COMPARISON OF SINGLE AND MULTI-STEP AIR RINGS Commercial, plant sizefilm equipment consisting of an extruder, die, air-ring assembly,water-cooled mandrel, external water bath, nip rolls and take-up rollswere used for the following test runs. A slip-ring adapter connectingthe extruder to the die permitted the die, ring assembly, mandrel andwater bath which were connected to one another in fixed relationship tobe slowly rotated in an oscillatory manner through an angle of 360 forthe purpose of randomizing any variation in film thickness. A speedcontrol on the oscillating equipment permitted the speed of rotation tobe varied from about 4 to inches per minute measured as lineal speed atthe periphery of the mandrel.

A standard batch of crystalline polypropylene resin having an n-heptaneinsoluble content of :1%, a melt index of 65:0.5 and a density in therange of 0902-0905 was used' for all of'the following tests.

TEST WITH MULTI -STEP RING The polypropylene resinwasfed to the extruderat the rate of 1000 pounds per hour and extruded through a 36 inch diehaving a die-gap of 2Qfmils (0.020 inches) at a temperature of 240x" C.land a pressure of 5,000 p.s.i.g., using an arrangement 9; die, rings,mandrel and water bath illustrated in the attached figures.

The extruded tube leaving the die at a lineal velocity of about 20 feetper minute was'lipartially inflated by air at a pressure in the range of0.05 to 0.5 inches of water issuing from the top edge of the mandrel,which had a maximum diameter near its top of about 46 inches, taperingto about 45 inches in one foot at its, bottom, and drawn down over thecooled mandrel through the water bath. A lower film bubble wasmaintained by air injected below the mandrel until the tube wasflattened by pinch or nip rolls and wound on take-up rolls.

Air was fed at a pressure ini the range of 0.05 to 1.5 inches of waterto the multi-step air-ring assembly which was disposed between the dieand mandrel and had the following inside diameters for the orificeplates: inner orifice plate-38.50 inches; middle orifice plate-42inches; outer orifice plate--45 inches. ,1

Under steady operating conditions shown below in Table I, draw-downspeeds of 300 lineal feet per minute and a film gauge thickness of 1 mil(0.001 inch) were readily obtaiined and yielded film having outstandingclarity and freedom from striations, fog or irregularity.

TA'BLE I.-STEADY STATE OPERATION WITH MULTI-STEP RINGS Distance of innerorifice plate from die inches 1.50 Distance of middle orifice plate fromdie do 3.125 Distance of outer orifice plate from die do 4.250

Air pressure on plenum chamber to rings-..in. H O..- 1.0 Air pressure inupper film bubble in. H O-.. 0.21 Air pressure in lower film bubbleAtmos. Air temperature C 20 Distance mandrel to die inches 6.0 Length ofmandrel do 12 Temperature of water to mandrel C..- 10 Temperature ofwater from mandrel C 15 Temperature of water to water bath C 10Temperature of water from water bath C..- 30 Oscillation speed (atperiphery of mandrel) inches per min 10 In order to obtain film of thehighest clarity, it is essential to maintain the film before quenchingin the water bath and cooling by contact with the mandrel above thetemperature at which crystallization occurs. Slow cooling by means ofair alone, for example, in the region where the film is in the form ofthe upper film bubble will cause fogging and opalescent zones to appear.vRapid quenching such as occurs in the water bath and on the mandrelyields film having the highest clarity.

TESTS WITH SINGLE AIR-RING The following tests, which in point of timepreceded the test with the multi-step air-ringsldetailed above, weremade with a commercial air-ring which consisted essentially of mountingplate 42 defining arr nnulus 76 through which air was fed. Difiiculty inproducing film of satisfactory quality was immediately apparent evenwhen the rate of polymer fed to the extruder was lowered to 250 poundsper hour and the temperature of extrusion was dropped so as to obtainfilm of higher rigidity. Attempts to utilize the 46 inch mandrel metwith failure and film of poor quality could only be produced with amandrel having a diameter of 38 inches.

Since the major difiiculty involved formation of an upper film bubblehaving a diameter in excess of the mandrel at a point below theair-ring, it was postulated that sealed guides which would direct thefilm by means of isolated areas of pressure exerted on the externalsurface of the bubble would overcome the difficulty. The result of manytrials in this direction are the multi-step rings of this invention.

What is claimed is:

1. In a process for the formation of tubular thermoplastic filmcomprising extruding a thermoplastic resin generally downward through adie in the form of a molten substantially tubular film body, partiallycooling said tubular film body by introducing a single stream of gasbetween said film and an air ring disposed outwardly from said film topartially solidify said film, downwardly passing said film to engage acooling mandrel positioned in a bath containing a cooling liquid andthereafter recovering said tubular film, the improvement which comprisesreducing the pressure of said single stream of gas indiscrete steps bymeans of a multi-step air ring surrounding said film wherein said ringincludes at least two annular orifices in echelon array havingsuccessively larger diameters between said die and said mandrel witheach orifice being independently adjustable as to its vertical distancefrom said die thereby defining a continuous passageway between said filmand said ring, in order to guide and control the expansion of said filmbetween said die and said mandrel.

2. Process according to claim 1 wherein said thermoplastic resin issubstantially crystalline in character.

3. Process according to claim 2 wherein said crystalline thermoplasticresin is polypropylene. 4. Process according to claim 2 wherein thetemperature to which said tubular film is preliminarily cooled in orderto partially solidify it prior to engaging said mandrel is to atemperature above the crystallization temperature of said thermoplasticresin and said cooling on said mandrel is below the temperature at whichcrystallization occurs in said resin.

5. An apparatus for extruding .film from thermoplastic compositionswhich comprises:

(a) a die, defining a die gap, for extruding a molten tubular body in agenerally downward direction;

(b) a liquid cooled mandrel of larger diameter than said die positionedaxially below said die;

(0) means for forwarding said tubular film from said dieldownwardly overthe outer surface of said mandre (d) a multi-step air ring meansdisposed outwardly from said tubular body and defining a continuouspassageway for a single stream of cooling gas between said ring meansand said body, said ring means including at least two annular orificesin echelon array having successively larger diameters between said dieand said mandrel, for reducing the pressure of said gas in discretesteps, each orifice being independently adjustable as to its verticaldistance from said die; and

(e) means for supplying a gas in a single stream under pressure betweensaid orifices and thereby to guide and control the expansion of the filmbetween said die and said mandrel.

6. Apparatus according to claim 5 wherein said orifices are verticallyadjustable to conform to a straight line projected through them, saidstraight line lying outside a straight line projected from said die-gapto the top edge of said mandrel.

7. Apparatus according to claim 5 wherein said orifices lie along an areprojected through them, the sagitta of said arc having a maximum of 7.5%of the length of the chord subtending the arc.

(References on following page) 7 8 References Cited 3,568,252 3/1971Masuda et a1 264-95 X UNITED STATES PATENTS 3,548,042 12/1970 Hmnchs26495 X 3,400,184 9/1968 Matsuo et a1. 26495 X JEFFERY THURLOW' PrimaryExamm" 3,507,006 4/1970 Princen. 5 US. Cl. X.R.

3,685,576 8/1972 North 425-72 X 264-95, 178 R, 209, 237; 425-71, 72, 326R

